The general result of inquiries into the size of the army after its radical re-organization by Diocletian and Constantine is that its total strength was between 600,000 and 650,000. This includes both the comitatenses and the limitanei, the mobile army and the stationary forces who garrisoned the exposed provinces. Of this total strength it is estimated that about one-third (more than 200,000) were in the mobile army and the rest in the garrisons. When you consider the large frontiers which had to be defended, the line of the Rhine, the line of the Danube, as well as the north frontier of Britain in the west, the long frontiers of Africa in the south, the Euphrates, and the Syrian desert in the east, the numbers seem very small.
Relatively to the lengths of the frontier the greater proportion of troops was demanded for the defence of the eastern frontier; for there the enemy was a mighty, well-organized state -the Persian Empire. On the western and northern frontiers the danger came from a number of independent barbarian peoples, who occasionally acted together, but were, even so, far outmatched by the Roman legions in discipline and drill. It has been commonly supposed, however, that this inferiority was more than balanced by their multitude, at least in the case of the East Germans, whose armies have been generally imagined to consist of hundreds of thousands. This idea is fundamentally erroneous, and it is one of the most important points to be quite clear about in studying the barbarian invasions.
The enormous figures for the German armies given by many of the chroniclers of the time are absolutely untrustworthy: not only are they on a priori grounds impossible, but they are inconsistent among themselves and inconsistent with the statements of those who were most likely to know. When we compare together the figures which we have good reason to consider trustworthy we reach a conclusion that the total number of one of the largest East German nations varied from 80,000 to perhaps 120,000, while that of the smaller peoples varied from 25,000 to 50,000. Now from these totals, which included women and children, the Germans could put a much larger fraction in the field than a civilised state. The military age began somewhat earlier and lasted much longer. A German host could number a quarter or a fifth of the population. And so we find that an army of one of the big East German peoples like the Visigoths, or Ostrogoths, or Vandals, would be as a rule about 20,000 or 25,000, or at most 30,000. And so in most of the battles between imperial troops and East Germans from the fourth to the sixth century we find that the opposing numbers were about 20,000 or so on either side. These facts put a different complexion on the whole history of the German invasions and conquests, and show that the problem of the military defence was not at all in itself hopeless or even superlatively difficult, and that if other elements had not entered in there was no reason why the Empire should have been dismembered. The numbers of the Germans did not make it inevitable.
These facts, as to the comparative size of the Roman armies and the German hosts which were opposed to them, are extremely important to grasp in following the course of the East German invasions, and in most histories they have been either passed over or misrepresented. The second important fact which should be emphasised is the gradually increasing power of the army and the consequent growth of German influence, which at first the Emperors did not realize as a danger. It was, in fact, a sort of peaceful penetration.
A Germanic element had been filtering into the population of the Empire, in certain districts, in other ways, in the first place, we must remember that the western fringe of Germany had been incorporated with the Empire, in the two Germanic provinces of Gaul. The imperial towns of Cologne, Treves, Mayence, were German. In the second place, many Germans had been induced to settle within the Empire as farmers in desolated tracts of country, after the wars of Marcus Aurelius in the second century. And there were settlements in the Belgic provinces of Germany who had come from beyond the Rhine and received lands in return for which they performed military service, and were organized in communities, and were technically called laeti. In many frontier districts there was a considerable German population; because lands were assigned to the soldiers who protected the frontiers (the limitanei), and as the army became more and more recruited from Germans, the population of a district on a military frontier might become largely German.
(Source: “The Invasion of Europe By the Barbarians”, by Bury, J.B.)
Research-Selection for NovoScriptorium: Anastasius Philoponus